2020-02-26 22:21:53

by Gustavo A. R. Silva

Subject: [PATCH] misc: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
int stuff;
struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[email protected]>
---
drivers/misc/mei/hw.h | 2 +-
drivers/misc/mei/mei_dev.h | 2 +-
drivers/misc/sgi-gru/grulib.h | 2 +-
drivers/misc/sgi-gru/grutables.h | 2 +-
4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/mei/hw.h b/drivers/misc/mei/hw.h
index 8231b6941adf..b1a8d5ec88b3 100644
--- a/drivers/misc/mei/hw.h
+++ b/drivers/misc/mei/hw.h
@@ -216,7 +216,7 @@ struct mei_msg_hdr {

struct mei_bus_message {
u8 hbm_cmd;
- u8 data[0];
+ u8 data[];
} __packed;

/**
diff --git a/drivers/misc/mei/mei_dev.h b/drivers/misc/mei/mei_dev.h
index 76f8ff5ff974..3a29db07211d 100644
--- a/drivers/misc/mei/mei_dev.h
+++ b/drivers/misc/mei/mei_dev.h
@@ -533,7 +533,7 @@ struct mei_device {
#endif /* CONFIG_DEBUG_FS */

const struct mei_hw_ops *ops;
- char hw[0] __aligned(sizeof(void *));
+ char hw[] __aligned(sizeof(void *));
};

static inline unsigned long mei_secs_to_jiffies(unsigned long sec)
diff --git a/drivers/misc/sgi-gru/grulib.h b/drivers/misc/sgi-gru/grulib.h
index e77d1b1f9d05..85c103923632 100644
--- a/drivers/misc/sgi-gru/grulib.h
+++ b/drivers/misc/sgi-gru/grulib.h
@@ -136,7 +136,7 @@ struct gru_dump_context_header {
pid_t pid;
unsigned long vaddr;
int cch_locked;
- unsigned long data[0];
+ unsigned long data[];
};

/*
diff --git a/drivers/misc/sgi-gru/grutables.h b/drivers/misc/sgi-gru/grutables.h
index a7e44b2eb413..5ce8f3081e96 100644
--- a/drivers/misc/sgi-gru/grutables.h
+++ b/drivers/misc/sgi-gru/grutables.h
@@ -372,7 +372,7 @@ struct gru_thread_state {
int ts_data_valid; /* Indicates if ts_gdata has
valid data */
struct gru_gseg_statistics ustats; /* User statistics */
- unsigned long ts_gdata[0]; /* save area for GRU data (CB,
+ unsigned long ts_gdata[]; /* save area for GRU data (CB,
DS, CBE) */
};

--
2.25.0

2020-02-27 06:52:18

by Winkler, Tomas

Subject: RE: [PATCH] misc: Replace zero-length array with flexible-array member

>
> The current codebase makes use of the zero-length array language extension to
> the C90 standard, but the preferred mechanism to declare variable-length types
> such as these ones is a flexible array member[1][2], introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning in case
> the flexible array does not occur last in the structure, which will help us prevent
> some kind of undefined behavior bugs from being inadvertently introduced[3]
> to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator may
> not be applied. As a quirk of the original implementation of zero-length arrays,
> sizeof evaluates to zero."[1]
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <[email protected]>
Ack, though be happier if this is would come in two separate patches.

> ---
> drivers/misc/mei/hw.h | 2 +-
> drivers/misc/mei/mei_dev.h | 2 +-
> drivers/misc/sgi-gru/grulib.h | 2 +-
> drivers/misc/sgi-gru/grutables.h | 2 +-
> 4 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/misc/mei/hw.h b/drivers/misc/mei/hw.h index
> 8231b6941adf..b1a8d5ec88b3 100644
> --- a/drivers/misc/mei/hw.h
> +++ b/drivers/misc/mei/hw.h
> @@ -216,7 +216,7 @@ struct mei_msg_hdr {
>
> struct mei_bus_message {
> u8 hbm_cmd;
> - u8 data[0];
> + u8 data[];
> } __packed;
>
> /**
> diff --git a/drivers/misc/mei/mei_dev.h b/drivers/misc/mei/mei_dev.h index
> 76f8ff5ff974..3a29db07211d 100644
> --- a/drivers/misc/mei/mei_dev.h
> +++ b/drivers/misc/mei/mei_dev.h
> @@ -533,7 +533,7 @@ struct mei_device {
> #endif /* CONFIG_DEBUG_FS */
>
> const struct mei_hw_ops *ops;
> - char hw[0] __aligned(sizeof(void *));
> + char hw[] __aligned(sizeof(void *));
> };
>
> static inline unsigned long mei_secs_to_jiffies(unsigned long sec) diff --git
> a/drivers/misc/sgi-gru/grulib.h b/drivers/misc/sgi-gru/grulib.h index
> e77d1b1f9d05..85c103923632 100644
> --- a/drivers/misc/sgi-gru/grulib.h
> +++ b/drivers/misc/sgi-gru/grulib.h
> @@ -136,7 +136,7 @@ struct gru_dump_context_header {
> pid_t pid;
> unsigned long vaddr;
> int cch_locked;
> - unsigned long data[0];
> + unsigned long data[];
> };
>
> /*
> diff --git a/drivers/misc/sgi-gru/grutables.h b/drivers/misc/sgi-gru/grutables.h
> index a7e44b2eb413..5ce8f3081e96 100644
> --- a/drivers/misc/sgi-gru/grutables.h
> +++ b/drivers/misc/sgi-gru/grutables.h
> @@ -372,7 +372,7 @@ struct gru_thread_state {
> int ts_data_valid; /* Indicates if ts_gdata has
> valid data */
> struct gru_gseg_statistics ustats; /* User statistics */
> - unsigned long ts_gdata[0]; /* save area for GRU data (CB,
> + unsigned long ts_gdata[]; /* save area for GRU data (CB,
> DS, CBE) */
> };
>
> --
> 2.25.0

2020-02-27 15:10:16

by Dimitri Sivanich

Subject: Re: [PATCH] misc: Replace zero-length array with flexible-array member

On Wed, Feb 26, 2020 at 04:22:40PM -0600, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <[email protected]>

Acked-by: Dimitri Sivanich <[email protected]>

> ---
> drivers/misc/mei/hw.h | 2 +-
> drivers/misc/mei/mei_dev.h | 2 +-
> drivers/misc/sgi-gru/grulib.h | 2 +-
> drivers/misc/sgi-gru/grutables.h | 2 +-
> 4 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/misc/mei/hw.h b/drivers/misc/mei/hw.h
> index 8231b6941adf..b1a8d5ec88b3 100644
> --- a/drivers/misc/mei/hw.h
> +++ b/drivers/misc/mei/hw.h
> @@ -216,7 +216,7 @@ struct mei_msg_hdr {
>
> struct mei_bus_message {
> u8 hbm_cmd;
> - u8 data[0];
> + u8 data[];
> } __packed;
>
> /**
> diff --git a/drivers/misc/mei/mei_dev.h b/drivers/misc/mei/mei_dev.h
> index 76f8ff5ff974..3a29db07211d 100644
> --- a/drivers/misc/mei/mei_dev.h
> +++ b/drivers/misc/mei/mei_dev.h
> @@ -533,7 +533,7 @@ struct mei_device {
> #endif /* CONFIG_DEBUG_FS */
>
> const struct mei_hw_ops *ops;
> - char hw[0] __aligned(sizeof(void *));
> + char hw[] __aligned(sizeof(void *));
> };
>
> static inline unsigned long mei_secs_to_jiffies(unsigned long sec)
> diff --git a/drivers/misc/sgi-gru/grulib.h b/drivers/misc/sgi-gru/grulib.h
> index e77d1b1f9d05..85c103923632 100644
> --- a/drivers/misc/sgi-gru/grulib.h
> +++ b/drivers/misc/sgi-gru/grulib.h
> @@ -136,7 +136,7 @@ struct gru_dump_context_header {
> pid_t pid;
> unsigned long vaddr;
> int cch_locked;
> - unsigned long data[0];
> + unsigned long data[];
> };
>
> /*
> diff --git a/drivers/misc/sgi-gru/grutables.h b/drivers/misc/sgi-gru/grutables.h
> index a7e44b2eb413..5ce8f3081e96 100644
> --- a/drivers/misc/sgi-gru/grutables.h
> +++ b/drivers/misc/sgi-gru/grutables.h
> @@ -372,7 +372,7 @@ struct gru_thread_state {
> int ts_data_valid; /* Indicates if ts_gdata has
> valid data */
> struct gru_gseg_statistics ustats; /* User statistics */
> - unsigned long ts_gdata[0]; /* save area for GRU data (CB,
> + unsigned long ts_gdata[]; /* save area for GRU data (CB,
> DS, CBE) */
> };
>
> --
> 2.25.0
>